The enteric nervous system (ENS) is the collection of neurons and support cells that reside in the gastrointestinal (GI tract). The ENS can exert autonomous control over most GI functions including motility. The ENS can accomplish this task because it composed of functionally distinct neuronal classes that release specific neurotransmitters. Therefore there must be unique mechanisms controlling transmitter from functionally distinct classes of neurons. This control may, be accomplished through the selective expression of presynaptic receptors or calcium channels by different classes of neurons and their nerve endings. Specific aim 1 will test the hypothesis that there are multiple neurotransmitter pools in enteric nerve endings and that the size and mobility of these neurotransmitter pools determine the strength of synaptic transmission at subsets of synapses. These studies will also test the hypothesis that 5-HT4 receptor agonists (drugs that stimulate intestinal motility) facilitate the movement of neurotransmitter from reserve to readily releasable pools. Specific aim 2 will focus on the functional role of a newly identified calcium channel subtype (R-type channels) in myenteric neurons. These studies will test the hypothesis that R-type channels are localized to the nerve endings of intestinal sensory neurons and that R-type channels mediate calcium entry required for the release of slow excitatory synaptic transmitters from sensory neurons but not for the release of fast or slow excitatory neurotransmitters from interneurons or motorneurons. These studies will also identify the calcium channels mediating release of neurotransmitters from interneurons and motorneurons. Finally, these studies will examine the contribution of R-type channels to neurotransmission underlying peristalsis in vitro. Specific aim 3 will test the hypothesis that that there are presynaptic nicotinic and P2X receptors on sensory nerve endings in the myenteric plexus. Data from these studies will contribute to our understanding of neuronal control of GI function. It is anticipated that our results will provide new insights into alterations in neural function leading to motility disorders. It is also anticipated that these studies will identify new targets for drugs that can be used to treat GI motility disorders.